It is well known in the art of producing chlorine and caustic soda by electrolysis of brine e.g. in a diaphragm cell, that the caustic solution produced therein contains an undesired high concentration of ClO.sub.3.sup.- (chlorate) ions.
It is also known to eliminate these chlorate ions by reducing them to chloride ions which may diffuse to the anode and be released together with the anodically produced chlorine gas.
U.S. Pat. No. 4,276,134 discloses a method of eliminating chlorate ions from the caustic solution by the employment of metallic iron, which is dissolved in the caustic solution sufficiently to reduce all existing chlorate ions, whereupon the contaminating iron (Fe.sup.2+ and/or Fe.sup.3+) is removed from the solution by electrodeposition of the iron onto a cathodically polarized substrate. In turn, the electrodeposited iron is again used to be dissolved in a fresh caustic solution to reduce chlorate ions. Even though the used iron is recycled, there are substantial losses of iron, which therefore has to be constantly added.
Typically the concentration of chlorate ions within the caustic solution being produced by electrolysis of brine is in the order of several hundred to a thousand ppm. The above U.S. patent reports a reduction of this concentration down to a few ppm.
To carry out the above procedure it is necessary to provide two different containers in which the two different steps of the described method (dissolving the iron and re-electroplating the dissolved iron ions) may be carried out, furtheron a rather complicated operation is employed to continually de-chlorate the produced caustic solution by circulating it from one container to the other and finally expensive inert anodes and from time to time replaceable cathodes for the electroplating are needed.
U.S. Pat. No. 4,436,599 discloses a process for producing an iron-activated cathode whereby the iron, which is applied to the cathode by adding to the cathode compartment within a chlor-alkali cell a certain amount of a ferrite solution, brings about a desired drop of the overvoltage.
However, this patent does not disclose the beneficial effect of iron in respect of the reduction of the chlorate concentration, and further, uses a procedure, whereby ferrite has to be constantly added to the cell contents, which contaminates the cell liquor and, consequently, has to be removed therefrom afterwards.